This invention relates generally to the acoustic logging of a subsurface formation penetrated by a borehole.
It is conventional practice to survey a subsurface formation surrounding a borehole by acoustic logging techniques in which acoustic signals are generated and received by a logging tool run through the borehole. One such survey involves the transmission and reception of directional acoustic signals, usually in a rotational mode about the borehole, in order to obtain an indication of the configuration of the borehole or the presence or absence of anomalies in the wall of the borehole or with the surrounding formations. Such an acoustic logging survey may be carried out utilizing separate transducers for transmission and reception, such as disclosed in U.S. Pat. Nos. 3,369,626; 3,371,319; 3,434,563; 3,434,568; 3,517,767; 3,728,672; 3,517,768; and 4,704,008, or by utilizing a single transducer, commonly termed a "transceiver", for both transmission and reception, such as disclosed in U.S. Pat. Nos. 3,406,779; 3,406,780; 3,503,038; 3,511,334; 3,518,679; 3,550,075; 3,553,640, and 3,668,619.
When utilizing separate directional transducers for transmission and reception it is necessary for best results to focus the transducers with regard to the formation being surveyed. For example, the transmitter and receiver are inclined toward one another in order that their respective directional transmission and reception paths meet on the wall of the borehole. While this presents no problem when logging a borehole of known and uniform diameter, in actual practice the borehole may vary considerably throughout the extent to which it is to be logged. Consequently, the use of separate transmitter and receiver oriented at a fixed focal distance may produce unacceptable results.
To avoid such an inherent disadvantage in the use of separate transmitting and receiving transducers, it has become the practice to utilize a single transducer, sometimes called a transceiver, for both transmission and reception of acoustic energy. The transducer is oriented within the borehole such that its directional transmission path is normal to the wall of the borehole. Since the transmitted acoustic energy strikes the wall of the borehole at an angle of incidence of zero or near zero, the reflected energy will travel along the same general path as the transmitted energy thus enabling a single logging run to be made in a borehole of varying diameter. This is the preferred method of logging whenever the intent is only to survey the borehole wall and the intent is not to survey beyond the borehole wall.
For investigating anomalies beyond the borehole wall, one problem encountered in logging with a single transducer for transmission and reception occurs in boreholes of high reflectivity or lined with well casing as is commonly done in hydrocarbon exploration and production. A strong primary acoustic reflection is developed from the normal incidence of the transmitted acoustic energy on the high reflectivity wall or well casing which, in combination with multiples of such reflection, interfere with the desired reflected acoustic signals from within the subsurface formations surrounding the borehole or from beyond the well casing.
In U.S. Pat. No. 4,255,798 to Havira an acoustic pulse technique for investigating casing and cement bond in a borehole is described wherein a plurality of transmitter/receiver transducers are used to direct acoustic pulses toward the casing and to receive the reflected signals. Each transducer is spaced around the circumference of the tool to examine different segments of the casing. The reflected signals consist of a large amplitude primary reflection caused by the acoustic discontinuity at the borehole fluid-casing interface followed by much smaller signal amplitudes representative of reverberations between the inner and outer walls of the casing. The amplitude of the smaller signal depends on the amount of energy leaked into the adjacent media. The total received signals are processed to determine the casing thickness or to evaluate the cement bond.
Commercial versions of this tool are described in papers entitled "Ultrasonic Cement Bond Evaluation" by R.M. Havira and published in the proceedings of the Society of Professional Well Logging Symposium, July 6-9 1982, paper N and "A Comparison of New Ultrasonic Cement and Casing Evaluation Logs with Standard Cement Bond Logs" by T. C. Sheives et al. and presented at the 61st Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Oct. 5-8, 1986, SPE paper 15436. The tools described have eight transducers spaced azimuthally to examine approximate 45 degree segments of the casing. None of the tools described measure directly the properties of the media surrounding the borehole. Only amplitudes and resonant frequencies are measured which are then related in some manner to the quality of the cement bond.
It is therefore an object of the present invention to provide for the acoustic logging of a borehole, particularly one lined with well casing, wherein the problem of borehole or casing primary and multiple reflections interfering with formation reflection signals is overcome or in the case of a cemented well casing the problem of primary and multiple reflections from the casing interfering with the imaging of voids and channels in the cement is overcome.